Traditional total hip replacements involve inserting a stem of a femoral implant into the medullary canal of a patient's femur after the femur has been resected at the distal end of the femoral neck. The stem is usually tapered such that its sides gradually converge from a wider proximal end to a narrower distal end. This configuration allows the stem to fill the majority of the medullary canal as the femur gradually narrows in a distal direction and this helps to anchor the implant in the femur. A rounded tip is provided at the distal end of the stem and a femoral neck and head is provided at the proximal end. The femoral head is constituted by a spherical ball configured for location within a corresponding acetabular cup.
In recent years, more conservative approaches, such as hip resurfacing, have been employed rather than total hip replacement as described above. In this case, the aim is to save as much healthy bone as possible and so the femur is preferably resected towards the proximal end of the femoral neck or even through the lower portion of the femoral head. The femoral head of the implant in the later case comprises a part-spherical exterior surface (configured for location within a corresponding acetabular cup) and an interior surface shaped to locate the femoral head on the remaining resected bone of the femur. This generally requires that the stem is relatively narrow at its proximal end so that there is room for sufficient healthy bone to be received within the profile of the femoral head to secure it in place. However, this configuration does not allow the stem to fill the majority of the medullary canal to anchor the implant in the femur and so cement is used to secure the femoral head on the femur.
Currently two diverging ideas lie behind the cement fixation of hip resurfacing femoral components.
The CONSERVE® Plus Total Resurfacing Hip System (C+) has a designed in 1 mm gap all around between the femoral head bone and the inside of the prosthetic component. For all prosthetic components fixed with cement a 1 mm cement mantle between the prosthetic component and the bone is often considered ideal for load transfer from the implant to the bone. High viscosity cement is intended for use in fixation of the C+ component. The cement is applied to the femoral head bone and an external pressurizing device (balloon type) is used to attempt to force cement into the bone (for micro-interlocking therewith) before insertion of the implant. The implant is then held in place on the femoral head until the cement has set (which takes approximately 12 minutes) before the femoral head is reduced into the acetabular component. In use, a major deficiency of the C+ system is medium and long term loosening of the femoral component.
The Birmingham Hip Resurfacing (BHR) component is designed to have no gap, with intended line-to-line contact, between the inside surfaces of the femoral component and the femoral head bone. In this case, low viscosity cement is used, no external pressurizing device is employed, and immediately following insertion of the BHR on the resected femur, the femoral component is reduced into the acetabular component before cement setting thereby not increasing the time taken for the operative procedure.
In use, loosening of the femoral component of the BHR is exceedingly rare. As the component is impacted into position, cement is driven into the bone of the femoral head giving excellent micro-interlock fixation. However retrieval evidence shows that large quantities of cement can be driven into the femoral head bone and rare instances of thermal necrosis of the femoral head bone have been described. This is a complication of an exothermic reaction due to cement setting made worse when large quantities of cement fill or substantially fill the femoral head bone.
Experience with fixing prosthetic components using bone cement over the past 50 years has shown that micro-interlock of cement into the spaces in cancellous bone is the desirable objective in order to achieve durable long-term component fixation. Between 1 mm and 3 mm of cement penetration into cancellous bone is usually considered ideal.
Three conditions for cement micro-interlock exist:
1) Suitable bone preparation. It is usual to prepare the surface of cancellous bone with pulsatile fluid lavage (usually saline) and brushing. Fat and other soft tissue are removed from the bone surface to allow cement to easily penetrate into spaces present in the cancellous bone.
2) Ideally low viscosity cement is used as this will easily penetrate into the cancellous bone spaces. However, a low viscosity cement must be contained as otherwise it will run out losing all potential for micro-interlock and leading to component loosening over time. For example, if low viscosity cement were used in the C+ component, the low viscosity cement would run out of the sidewall gap leading to certain component loosening. For this reason high viscosity cement, which does not run out, is used with the C+ implant. However, high viscosity cement has a lower potential to achieve cancellous bone micro-interlock. Some surgeons who use the C+ implant attempt to use low viscosity cement with an external pressurizing device to achieve bone micro-interlock, then they wait for partial cement curing (setting) allowing the cement to reach a higher viscosity state before femoral component insertion. This requires very precise timing by the surgeon and if the cement is allowed to cure too much the component cannot be inserted; too little curing and the cement runs out losing all fixation.
3) Pressure must be applied to the low viscosity cement in order to induce flow of cement into the cancellous bone spaces. In femoral head resurfacing this pressure can either be applied by an external pressurizing device or by using the component design in conjunction with the reproducible bone cuts from the usual (known art) size specific resurfacing bone cutting instruments. In the bone preparation of the femoral head for a 50 mm component, different instruments are used from those used for bone preparation of a size 48 mm or a size 52 mm component. An external pressuring device is awkward for the surgeon to use, is expensive and requires considerable surgeon skill and patience to achieve the desired objective.
It is therefore an aim of the present invention to provide an improved femoral head resurfacing implant which ameliorates some or all of the above-mentioned problems.